Code division multiple access (CDMA) is a technique for spread-spectrum digital communications used for many applications, including, for example, mobile communications. In CDMA systems, data signals are combined with a spreading waveform to form a coded signal for transmission. At the receiver, the received signal is combined with a similar spreading waveform to extract the data signals. The technique provides high data capacity by spreading signal energy over a wide bandwidth to increase bandwidth utilization and reduce the effects of narrow band interference. Multipath effects make synchronization more difficult since the wireless channel from a base station to a reception device may have several paths of different time-delays, which may vary due to the movement of the reception device.
Wireless communication devices (e.g., mobile stations), which communicate CDMA signals with remote base stations, often operate in a site selection diversity transmit (SSDT) mode for selection of base stations to communication with. One problem with SSDT mode is that some algorithms, such as power control and frequency estimation algorithms, are unable to quickly adapt to the quick changes in channel conditions that occur when transferring communications from one base station to another. This may result in poor performance, especially after the selection of a new primary base station for communication.
One specific problem area is power control. Mobile stations conventionally implement power control methods to minimize transmission power while maintaining desired performance levels. Conventional power control methods implement a nested loop structure having an outer loop and an inner loop to control transmit power. In the outer loop, the block error rate (BLER) of received data may be monitored and compared to a desired BLER. A signal to interference ratio (SIR) target is then developed for the receiver based on the comparison. In the inner loop, a measured SIR for a received signal is compared to the SIR target. A power control message may then be generated for delivery to the transmitter based on the result of the SIR comparison (e.g., indicating whether transmit power modifications are desirable). A problem with this power control technique (and other similar methods) is that the BLER measurement is relatively slow and, therefore, the SIR target is not able to adapt quickly to changes in channel conditions. As may be appreciated, this may result in poor communication quality after a new base station is selected, until the transmit power may be readjusted.